Influence law and control mechanism of overburden isolated grout injection on methane emission in the working face

Abstract

Overburden isolated grout injection (OIGI) can well reduce surface subsidence, waste emission, and coal resource waste while preserving the ecological integrity of mining areas, thereby becoming the development direction of green coal mining in the future. However, its influence law and control mechanism on methane emission under coal seam mining remain unclear. In light of this, data including methane emission and surface subsidence during the mining of the 22301 working face in Tunlan Mine was collected by on-site measurements through online methane extraction monitoring systems, leveling instruments, and electronic total stations. The collected data and geological information were subject to numerical simulation and theoretical analysis to explore the methane emission law and its controlling factors after OIGI. The results demonstrate the following: (1) The total methane emission decreases by 46% after the implementation of OIGI, primarily due to reduced methane emission from the adjacent seams into the mining coal seam. (2) After OIGI, the vertical stress on the coal and rock strata in goaf rises, while the stress on the surrounding coal pillars decreases; the stress in the whole stope tends to be distributed more uniformly. (3) A relationship between the overburden stress and permeability was established, which reveals that stress plays a major role in controlling methane emission from adjacent seams into the goaf. After OIGI, the rising stress on the overburden above the goaf reduces the porosity and permeability, narrowing methane migration pathways, consequently reducing the volume of methane emission from surrounding seams into the coal seam. Stress redistribution after mining gives rise to methane rivers and low-permeability walls around the goaf, the volumes of which decrease as the grouting pressure rises. Methane rivers are where methane accumulates, and pre-drainage boreholes from adjacent seams should be directed towards these methane rivers to achieve enhanced methane extraction. The conclusions of this study provide theoretical support for efficient methane extraction under OIGI mining.